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Are these aftermarket speakers? I'm having a hard time picturing the plastic housing your referring to. Are they surface mount type speakers that hhave been added on?
Do your rear seat backs fold down?
Here's some instructions I found that may help you.
REMOVAL & INSTALLATION
See Figure 1
Remove the speaker cover on the door or back dash.
Remove the attaching screws for the speaker. Note location of speaker wires and .
Pull the speaker from its mounting surface.
Fig. 1: View of the speaker locations found on Corollas
Attach the speaker wiring to the back of the unit.
Position the speaker into its mounting surface and secure with the screws.
Secure the speaker cover. Check radio system for proper operation.